1. Field of the Invention
The present invention relates to a gas friction pump including a housing having suction and gas outlet openings, and rotor and stator components arranged in the housing for delivering gases and for obtaining a pressure ratio.
2. Description of the Prior Art
For delivery of gases, gas friction pumps of different types are used. Their operation is based on transmission of pulses from movable walls to gas particles. In this way, a gas flow in a predetermined direction is achieved. Gas friction pumps, which operate in a pressure range in which the mean free path length of gas molecules, i.e., the mean travel path of the gas molecules, is large in comparison with the pump dimensions, are called molecular pump.
A first gas friction pump was described by Gaede (Ann. Phys. 41, 1913, p. 337). Further technical developments, with retaining the basic principle, were made by Siegbahn (Arch. Math. Ast. Phys. 30B, 1943), Holweck (Comptes Reduc Acad. Science 177, 1923, p. 43), and Becker (Vakuum Technik 9/10, 1966). The molecular pump of Becker is known as a turbomolecular pump, and it has found wide application in science and technology. The present invention is based on the turbomolecular pump.
The drawbacks, which characterize the conventional turbomolecular pumps and the elimination of these drawbacks according to the present invention, are applicable for other gas friction pumps as well.
The suction capacity of a turbomolecular pump essentially depends on the inlet cross-section of the suction flange, on the mean circumferential speed of the blade crowns of the rotor blades adjacent to the pump-out space, and on the pump structure. In addition, the suction capacity depends on the internal structure of the pump, gradation of the pressure ratio, and the suction capacity or speed between separate pump stages. Lastly, but not the least, the suction capacity depends on which part of the pump or the pump combination works against the atmosphere.
These characteristics can be optimally established, and the rotational speed can be increased, within the area of technical possibilities, to an extent that the largest portion of the molecules, which are collected on the blade crowns, can be pumped out. However, these molecules do not include all of the molecules entering the inlet cross-section of the suction flange. A large area of this surface is formed by the end surface of the rotor which does not have a gas delivery structure. Even when the blade crown is increased further, at the expense of the rotor end surface, the suction capacity is still limited by the cross-section of the suction flange. The quantity of the pumped molecules depends on how many molecules reach the gas delivery structure of the inlet stage. Even of that quantity which reaches the gas delivery structure, a definite number of molecules is rebound and, therefore, is not acquired by the delivery mechanism.
Accordingly, an object of the present invention is to provide a gas friction pump having a noticeably greater suction capacity than conventional pumps, with the same cross-section of the suction flange as the conventional pumps.
This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing an additional, at least one-stage, concave pump unit provided at a side of the suction opening and having a gas delivery structure, with the additional pump unit being so formed that gas delivery takes place in a radial direction, and with rotor components of the additional pump unit being supported on the same rotor shaft as the rotor components of the gas friction pump itself.
With the additional pump unit according to the present invention, which has at least one stage but may have several stages, the largest part of the molecules, which are rebound of a gas delivery structure, are reflected onto another region of this structure and, thus, still reach the delivery mechanism. This effect is achieved by providing an additional pump unit having a concave shape, which enables delivery of molecules in the radial direction. As a result, the reflected or rebound molecules are recaptured and are delivered further. This substantially increases the suction capacity with the same suction cross-section.
A further advantage of the present invention consists in that the concave suction space provides room for components of different recipients, permitting to achieve an effective pumping action.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.